1. Field of the Invention
The present invention relates to a solid tissue impedance estimating method, a cardiac output calculating method, a pulmonary artery wedge pressure calculating method, a cardiac output monitoring device, a cardiac output monitoring system, a pulmonary artery wedge pressure monitoring device and a pulmonary artery wedge pressure monitoring system.
This application is based on Japanese Patent Application No. 2006-333691, the content of which is incorporated herein by reference.
2. Description of Related Art
If changes in cardiac function can be continuously monitored in patients with heart failure who are implanted with devices such as implantable defibrillator or cardiac resynchronization device, it becomes possible to (1) adjust therapy in response to each patient's condition, and (2) detect cardiac deterioration earlier and initiate therapy sooner. As a result, patient prognosis or quality of life (QOL) indicators such as duration of hospital stay, etc. can be improved. For this reason, cardiac function monitors that can be incorporated into implantable devices are necessary from a clinical viewpoint.
Measuring and evaluating both cardiac output and pulmonary artery wedge pressure are generally known as necessary techniques for evaluating cardiac function. Examples of conventional techniques for measuring or estimating cardiac output include those disclosed in the Publications of U.S. Pat. No. 4,450,527, U.S. Pat. No. 5,417,717, U.S. Pat. No. 5,058,583 and U.S. Pat. No. 6,438,408. Examples of conventional techniques for estimating pulmonary artery wedge pressure or the degree of pulmonary congestion include those disclosed in the Publications of U.S. Pat. No. 6,473,640, U.S. Pat. No. 6,595,927, and U.S. Pat. No. 6,829,503.
The method disclosed in the Publication of U.S. Pat. No. 4,450,527 measures the impedance between electrodes placed on the body surface. As a result, electrode position and electrode impedance can vary due to body movement or perspiration, so that an accurate estimate of cardiac output is not possible. Moreover, since it is not realistic to attach electrodes on the body surface for an extended period of time, this method is not suitable for long-term monitoring.
The methods disclosed in the Publications of U.S. Pat. No. 5,417,717 and U.S. Pat. No. 5,058,583 estimate cardiac output from changes in right ventricular volume using an electrode placed within the right ventricle. However, the accuracy of these estimates has not been proven. The method in the Publication U.S. Pat. No. 6,438,408 estimates cardiac output from changes in left ventricular volume using an electrode inserted within the left ventricle. However, since the electrode remains within the patient's left ventricle for a long time, problems such as thrombosis and infection occur. Accordingly, it is difficult to put this method to practical use.
The methods disclosed in the Publications of U.S. Pat. No. 6,473,640 and U.S. Pat. No. 6,595,927 measure the time-averaged impedance between a can electrode implanted in the thoracic wall and a lead in the right ventricle or the right atrium, to estimate the degree of pulmonary congestion. Accordingly, changes in the right ventricular or right atrial volume are partially included in the signals. As a result, if there is a dilatation of the right ventricular volume, this can be misinterpreted as pulmonary congestion, even when no pulmonary congestion is present, i.e., even when there is no abnormality in the pulmonary artery wedge pressure.
The method disclosed in the Publication of U.S. Pat. No. 6,829,503 estimates the degree of pulmonary congestion by measuring the time-averaged impedance of the lungs, using a can electrode embedded in the thoracic wall. In this method, the pulmonary circulatory blood volume is estimated from the time-averaged pulmonary impedance signal, and the degree of pulmonary congestion is estimated. However, theoretically, pulmonary artery wedge pressure or the degree of pulmonary congestion is determined not only by the pulmonary circulatory blood volume but also by the cardiac output. For example, it is known that the pulmonary artery wedge pressure increases when the cardiac output becomes lower, even if the pulmonary circulatory blood volume is the same. The method disclosed in the Publication of U.S. Pat. No. 6,829,503 does not take changes in cardiac output into account, so that the estimate of the degree of pulmonary congestion is not accurate.
Further, in each of the cases of the Publications of U.S. Pat. No. 6,473,640, U.S. Pat. No. 6,595,927, and U.S. Pat. No. 6,829,503, it is not possible to estimate the values of pulmonary artery wedge pressure.